The present invention relates to electronic structures in Group III nitrides, and in particular relates to an improved structure for gallium nitride (GaN) based devices, particularly light emitting diodes (LEDS).
Gallium nitride and the related binary, ternary, and quaternary Group III nitride compounds have become wide bandgap materials of choice for light emitting diodes (including diodes that emit in the green, blue, and ultra-violet portions of the spectrum). In addition to their characteristic wide bandgaps, which permit UV transitions as well as those in the high energy blue and green portions of the visible spectrum, the Group III nitrides are direct emitters; i.e., the recombinations that generate photons avoid losing energy to vibrational transitions during the quantum event. As a result, Group III nitride-based devices can provide an efficiency advantage over those formed in other wide bandgap materials such as silicon carbide (SiC).
As well recognized by those of ordinary skill in this art, the term “diode” refers to a device (in this case a light-emitting device) that has adjacent n-type and p-type portions that define a p-n junction or its functional equivalent. Under an applied potential difference (voltage), current can flow across the junction to generate the recombinations of holes and electrons that in turn generate the photons. Accordingly, light-emitting diodes that incorporate Group III nitrides must incorporate, in most circumstances, both n-type layers of the nitride and p-type layers of the same or a different nitride. Junctions or their equivalent between identical Group III nitrides (e.g., p-type GaN and n-type GaN) are generally referred to as homojunctions, while those between two different nitrides (e.g., GaN and AlGaN) are referred to as heterojunctions. When a three-layer structure includes three different types of Group III materials, they are referred to as double heterojunctions. Other structures useful in diodes include multiple quantum wells as well as superlattices. All of these structures are well understood in this art. Exemplary patents and applications describing the basics of these devices as well as particular features include, for example, commonly-assigned U.S. Pat. Nos. 6,201,262; 6,187,606; 6,120,600, 5,838,706; 5,739,554; 5,592,501; 5,393,993 and 5,210,051.
As an understanding of Group III nitride devices has developed, it has generally been the case that the p-type layers have been more difficult to activate than are the n-type layers. As used herein and as well understood by those in this art, the terms “activate” or “activation,” refer to the characteristic of dopant atoms in a crystal in which they can functionally affect the electronic aspects of the device. Stated differently, if dopant atoms (such as magnesium, which acts as a p-type dopant in Group III nitrides) are physically present in the gallium nitride, but not located properly within the crystal structure (typically interstitially rather than in lattice positions), they will behave (at least electronically) as if they were simply absent. For example, a p-type Group III nitride layer with an activated concentration of magnesium of 1×1017 cm−3 may have a total concentration of magnesium atoms of 1×1020 cm−3.
Thus, one of the steps generally required in the production of semiconductor materials and devices, including Group III nitride devices, is to build or treat the device or structure in such a way that the desired number of dopant atoms become activated.
As noted above, such activation has been difficult for magnesium in p-type Group III nitrides, particular GaN. For example, relatively early patents in this field to Nakamura, Nos. 5,468,678 and 5,306,662, describe a step of annealing a p-type layer to attempt to reach the proper activation. Various hypotheses have been forwarded to explain this scenario, many of which assume that magnesium (Mg) is easily bound to hydrogen (H) and thus prevented from becoming activated in p-GaN. Thus, the annealing step has been assumed to serve the purpose of dissociating hydrogen from magnesium and thus allowing the magnesium to become activated (e.g., Column 8, lines 13-36 of No. 5,306,662).
More recently, however, device performance has indicated that the annealing step, standing alone, may not always create activation in the manner intended or previously hypothesized.
Conventionally, the LED structure is formed by starting with a silicon carbide substrate, and then adding an appropriate buffer layer. Epitaxial layers of n-type gallium nitride and p-type gallium nitride, or appropriate Group III nitride multiple quantum wells (MQW's) are added on the buffer to form the active portion of the device. Then, silicon dioxide is added to the active layer or layers to protect the p-type gallium nitride layer during further processing steps, many of which include heating the structure.
As part of the development of these devices, it was determined and decided to avoid the use of nickel for the necessary ohmic contacts in order to (among other reasons) permit lower temperatures to be used during other steps. Higher-temperature process steps can negatively affect gallium nitride, which has a dissociation temperature of about 1250° C. In this regard, an ohmic contact can be formed (and potentially annealed) at lower temperatures by using a silicon carbide (an advantageous substrate material for Group III nitride devices) substrate implanted with additional doping on the contact side. This technique is set forth in commonly assigned U.S. Pat. Nos. 5,409,859 and 5,323,022, the contents of which are incorporated entirely herein by reference.
Accordingly, success in formic ohmic contacts with these lower temperature processes encouraged the elimination of the silicon dioxide step as no longer being necessary to protect the GaN (or other Group III nitride) layers from exposure to high temperatures. Eliminating a theoretically unnecessary process step is, of course, entirely consistent with conventional engineering thought in that using fewer steps and lower temperatures should be advantageous for the process and the end device.
GaN-based LEDs formed in this manner appeared to be satisfactory until a protective layer of silicon nitride was added for passivation purposes as the last protective layer. In such devices, the forward voltage would increase dramatically and undesirably. Although higher voltage can be desirable in some cases, a forward voltage that is excessive with respect to a particular material or structure can overheat and damage or destroy devices such as LEDs.
From a positive empirical standpoint, devices made using the silicon dioxide layer step exhibited higher effective doping activation than those devices made without the SiO2 layer.
Accordingly, a need exists for a better understanding of the function of the anneal and of activation in p-type Group III nitrides, particularly gallium nitride, and for improving the manufacture and structure of p-type Group III nitride and GaN layers in such devices.